LS is incredibly useful. Honestly as an AST main, whenever I switch to WHM I end up forgetting that they don't have an insta-cast spell and it puts me into some bad situations. :P

WHM does have a comparable skill, Presence of Mind. It lasts 5 seconds longer than LS with the same CD, but it doesn't reduce your damage output, and it also decreases your spell recast time. LS reduces your basic attacks and magic attacks by 25%, so maybe that's a trade-off. WHM also has better healing numbers overall in that helps.
It feels more like a thematic thing imo. AST is pretty much a time mage, so being able to use a spell that gives you instant casts for a short while makes sense.

As for LS with Ascend, a Swiftcast combo/macro is still preferable. The only time I'll use LS specifically for Ascend is if I've just rezzed and we severely need a second. It cuts the time from 8s to 5.5s, so it's still a longer cast time than is preferable in most situations.